One of the most important characteristics of a winter tyre is its grip on snow (i.e. a snow-covered surface), and therefore its ability to generate sufficient friction as it rolls over the snow-covered surface. The total friction generated between a tyre and a snow-covered surface is the sum of a digging force—produced by the tyre tread penetrating the snow—and a shearing force—produced by friction between the snow trapped by the tyre tread (in particular, in the tread grooves) and the snow on the snow-covered surface. To increase the shearing force, it is necessary to increase the amount of snow trapped (gripped) by the tyre tread, so the tread has a number of normally zig-zag-patterned sipes extending radially inwards of the blocks from the rolling surface.
The presence of the sipes, however, also has negative aspects, in that, while improving performance on snow or ice, it also impairs performance on dry or (to a lesser extent) wet surfaces. More specifically, the presence of the sipes greatly increases the flexibility of the tread blocks transversely (i.e. axially, i.e. parallel to the rolling axis of the tyre), and longitudinally (i.e. circumferentially, i.e. tangent to the tyre circumference), and so greatly increases deformation of the tread blocks when subjected to transverse forces (when cornering) or longitudinal forces (when braking or accelerating). Severe deformation of the tread blocks by transverse or longitudinal forces is invariably negative, by reducing the contact area between the rolling surface (i.e. the outer surface of the tread blocks) and the road surface, thus reducing the total force the tyre is able to transmit to the road surface.
Patent Application EP1669217A1 describes a winter tyre tread band comprising: a number of longitudinal and transverse grooves, which define a number of blocks, each projecting radially upwards from a base surface of the tread band; and a number of sipes formed in at least some of the blocks, and each extending radially inwards of a corresponding block from a top surface forming a rolling surface of the tread band. In Patent Application EP1669217A1, each sipe is formed starting with a wave pattern in a plane defined by the longitudinal axis Tc and transverse axis Tw, and by moving this wave pattern along the radial axis Tr and at the same time in undulating fashion along the longitudinal axis Tc only.
The sipes described in Patent Application EP1669217A1 increase the rigidity of the blocks along the longitudinal axis Tc and transverse axis Tw, by having a dual wave pattern in both the plane defined by the longitudinal axis Tc and transverse axis Tw, and the plane defined by the longitudinal axis Tc and radial axis Tr.
Patent Application EP1987964A1 discloses a pneumatic tire including blocks on its tread which are segmented by plural circumferential grooves extending along a tire circumferential direction and plural lateral grooves extending along a tread width direction; circumferential sipes extending along the tire circumferential direction are provided in the blocks. Inside circumferential sipes extend straight along a tire radial direction and outside circumferential sipes extend along the tire radial direction in zigzag patterns.
Patent Application EP1987964A1 discloses a pneumatic tire including blocks on its tread which are segmented by plural circumferential grooves extending along a tire circumferential direction and plural lateral grooves extending along a tread width direction; circumferential sipes extending along the tire circumferential direction are provided in the blocks. Inside circumferential sipes extend straight along a tire radial direction and outside circumferential sipes extend along the tire radial direction in zigzag patterns.